Optical microscope (OM) is widely used in scientific research, medical field and industrial production and so on. The advantages of OM observation mainly include: the convenience of specimen preparation, simple operations, adaptability to outside environment, diverse ways of observation and so on. However, when the scale of the objects goes down to less than half of the wavelength (about 200 nm), OM is no longer applicable due to light diffraction. In order to obtain a higher resolution, the scanning electron microscope (SEM) was invented in 1960's, and the resolution of SEM can reach to several nanometers even sub-nanometer scale.
However, there are many disadvantages with SEM, among which the complexities of specimen preparation and rigorous testing environment are the main factors precluding its popularity. For example, the objects to be observed are always put into a high-vacuum chamber, therefore many treatments to the specimens are necessary such as sampling, drying, freezing or coating. Therefore, there are many objects hard to observe with conventional SEM, including huge object, liquid specimens, in-vivo biological specimens and so on.
In order to solve the above problems, new kinds of SEMs or novel accessory structures for SEM have been developed, in which the environmental scanning electron microscope (ESEM) is a milestone. The original idea of ESEM called atmospheric scanning electron microscope (ASEM) is shown in the U.S. Pat. No. 4,596,928 with the title “method and apparatus for an atmospheric scanning electron microscope”. After that ESEM has experienced a rapid development, the development relates to specimen preparation, detection method, and signal processing and so on.
Different from conventional SEM with a high-vacuum chamber (a typical pressure of less than 10−5 Torr), the ESEM usually has a gaseous environment with a high pressure typically between 0.1 Torr and 50 Torr. In this condition the specimen can be moist objects (such as biological specimens), non-conductive objects.
However, these ESEMs still have an enclosed space (specimen chamber or environmental cell), therefore observation is still difficult when it comes to huge specimen hard for sampling, or when frequent specimen exchange is necessary in line operations, which is common in industrial manufacture. SEM without specimen chamber is proposed to give an answer to this question. U.S. Pat. No. 4,596,928 with the invention called atmospheric scanning electron microscope (ASEM) provides the basic idea of SEM used in the open air without a specimen chamber. However, the key problem for SEM in the open air is that the mean free path (MFP) of electrons in the air is very small (tens or hundreds of micrometers), therefore the working distance of SEM is also very small that makes the SEM inapplicable. Besides how to create a gaseous environment and controlling the pressure in an open region without a chamber is also challenging. In the patent with Pub. No. US 2015/0380207 A1, one or more conduits are used to inject a gas, however, the pressure of the local gaseous environment is not stable and controllable. In addition, the pressure of the injected gas in the specimen region is a positive pressure, and hence the MFP will be smaller than that in one condition. All of these make this method very difficult in practical usage.
In conclusion, when the SEM is used in the open area without a specimen chamber, several problems need to solve include: how to create a local gaseous environment; and how to control the pressure of the area.